1. Field of the Invention
The invention relates to an electric power steering system.
2. Description of the Related Art
Power steering systems for a vehicle include an electric power steering system (EPS) that uses a motor as a drive source. In an EPS, usually, a control unit thereof detects a steering torque that is transmitted via a steering shaft, and calculates a target assist force that should be applied to a steering system based on the steering torque. Then, the control unit executes a feedback control so that an actual current value is brought to a current command value that corresponds to the target assist force. Thus, the control unit controls an operation of an actuator by adjusting a drive current that is applied to a motor.
In the EPS, an increase in quietness is one of the most important issues to be addressed. Examples of the situations where an increase in quietness is especially demanded include a situation where a steering operation is performed when a vehicle is traveling at a low speed, and a situation where a steering operation is performed when the vehicle is not traveling, that is, a steering wheel is turned by a large amount when the vehicle is not traveling.
When the vehicle is traveling at a low speed, a road reaction force is large. Therefore, a higher torque is required to perform a steering operation. Therefore, noise and vibration are likely to be generated due to an operation of a motor used as a drive source. Especially when the vehicle is not traveling, a steering wheel is turned relatively slowly. Therefore, a driver easily realizes generation of the noise and vibration.
In order to address the above-described problem, Japanese Patent No. 3231932 describes a technology in which a feedback gain is changed based on a vehicle speed to decrease the response of a control. Similarly, Japanese Patent Application Publication No. 2001-239947 (JP-A-2001-239947) describes a technology in which a feedback gain is changed based on a steering speed (rotational speed of a steering wheel).
One of the major factors of the above-described noise and vibration is a ripple component that is contained in the detected steering torque and that is produced due to configurations outside the motor that is used as the drive source, for example, changes in the states of the mesh of speed reduction gears. A target assist force is calculated based on the steering torque that contains the ripple component, and the feedback control is executed based on a current command value that corresponds to the target assist force. Then, the ripple component is amplified and the above-described noise and vibration are generated. Accordingly, as described above, if the feedback gain is changed to decrease the response of the feedback control, amplification of the ripple component is suppressed. According to the technologies described in the above-described documents, the situation where abnormal noise and vibration are likely to become obvious is estimated based on a vehicle speed or a steering speed and the response of the feedback control is decreased. In this way, quietness is increased while a good steering feel is maintained.
However, according to the technologies described in the above-mentioned documents, it is estimated that abnormal noise and vibration are likely to become obvious when the vehicle is traveling at a low speed or when a steering speed is low. However, even when the vehicle is traveling at a low speed or when the steering speed is low, vibration and abnormal noise are sometimes less likely to be generated. For example, on a road having a low friction coefficient μ, a detected steering torque is a low value because a road reaction force is low. Therefore, the influence of the ripple component contained in the steering torque is limited. In such a case, if the response of the feedback control is decreased, the steering wheel is not returned appropriately and a steering angle is not decreased to a desired value. This results in deterioration of a steering feel. Therefore, there is still a room for improvement of the steering feel.
Japanese Patent Application Publication No. 2008-6919 describes a technology in which a feedback gain is changed based on a value of electric current actually applied to a motor. However, even if the level of detected actual current value is high, noise and vibration are sometimes less likely to become obvious. Therefore, an adverse effect of a decrease in the response of a control may be caused, as in the technologies described in the above-mentioned documents.
Japanese Patent Application Publication No. 2006-131191 (JP-A-2006-131191) describes a technology in which characteristics of a phase compensation control that is executed on a detected steering torque are changed based on the ratio of a change in a base assist component to a change in a steering torque. Thus, a steering feel is improved. However, a frequency band that is a target of the phase compensation control differs from a frequency of a ripple component contained in the steering torque. Therefore, the technology described in JP-A-2006-131191 does not solve the above-described problem, either.